1. Field of Invention
The present invention relates to an iron-based sintered alloy with high performance and low cost for use as a valve seat of all internal combustion engine. The present invention also relates to a production method of the iron-based sintered alloy.
2. Description of Related Art
There is a tendency of increasing thermal load and mechanical load, to which the valve seat of an engine is subjected, along with the performance increase of an internal combustion engine as increasing the fuel efficiency and reducing an exhaust emission. In order to cope with this tendency, the sintered alloy to be used as valve seats has been strengthened by means of high alloying, forging, or copper infiltration. For example, chromium (Cr), cobalt (Co) and tungsten (W), which are added in the raw material powder for producing the iron-based sintered alloy, enhance the high-temperature strength of the alloy. Copper infiltration enhances the thermal conductivity of the sintered compact and hence indirectly enhances the high-temperature strength. Meanwhile, the strengthening of the sintered alloy by means of high-pressure compacting, powder forging, cold forging and high-temperature sintering are effective for increasing the mechanical strength of the sintered compact.
The present applicant proposed the iron-based sintered alloy, which consists of an iron base matrix with nickel (Ni)-molybdenum (Mo)-chromium (Cr)-carbon (C) and hard particles dispersed in the matrix, in Japanese Unexamined Patent Publication (kokai) No. 09-053158 (hereinafter referred to as xe2x80x9cprior applicationxe2x80x9d). However the proposed alloy is expensive since the matrix contains a large amount of expensive alloying elements. In the prior application, the performance of a valve seat is evaluated in terms of valve clearance between a cam and a cam follower. The valve clearance is mainly the total wear of the valve seat and the valve which are subject to hammering and sliding wear. The present inventors paid attention to the respective parts subject to the hammering and sliding wear and made further researches and discovered that high-alloying can be avoided.
Copper infiltration into the internal poles of the sintered compact enhances the thermal conductivity, so that the temperature of the material is not liable to rise even when the combustion temperature becomes high. Wear-resistance at high temperature is thus enhanced and the usable temperature of the iron-based alloy is increased. However, the copper-infiltrated sintered alloy needs secondary sintering, which increases the production cost.
It is, therefore, an object of the present invention to provide an iron-based sintered alloy, in which the alloying elements are reduced to the minimum level, for use as a valve seat of an internal combustion engine.
It is also an object of the present invention to provide a method for producing an iron-based sintered alloy for use as a valve seat of an internal combustion engine without secondary treatment such as copper infiltration.
In accordance with the objects of the present invention, there is provided an iron-based sintered alloy, which consists, by weight %, of from 0.5 to 5% of nickel (Ni), from 0.5 to 4% of chromium (Cr), from 0.5 to 2% of carbon (C), the balance being iron (Fe) and unavoidable impurities, and which has a microstructure comprising an iron-based matrix containing the nickel (Ni) and a part of the chromium (Cr) as solutes and carbides containing the other part of the chromium (Cr) and dispersed in the iron-based matrix. This alloy is hereinafter referred to as the Fexe2x80x94Nixe2x80x94Crxe2x80x94C alloy.
The iron-based sintered alloy according to the present invention may additionally contain one or more of the following hard particles.
(1) Hard particles which consist, by weight %, of from 50 to 57% of chromium (Cr), from 18 to 22% of molybdenum (Mo), from 8 to 12% of cobalt (Co), from 0.1 to 1.4% of carbon (C), from 0.8 to 1.3% of silicon (Si) and the balance being iron (Fe).
(2) Hard particles which consist, by weight %, of from 27 to 33% of chromium (Cr), from 22 to 28% of tungsten (W), from 8 to 12% of cobalt (Co), from 1.7 to 2.3% of carbon (C), from 1.0 to 2.0% of silicon (Si) and the balance being iron (Fe).
(3) Hard particles which consist, by weight %, of from 60 to 70% of molybdenum (Mo), 0.0% less of carbon and the balance being iron (Fe).
(4) Hard particles which consist of Stellite alloy
The hard particles are in an amount of from 3 to 20% by weight based on the iron-based sintered alloy, i.e., total of the Fexe2x80x94Nixe2x80x94Crxe2x80x94C alloy and the hard particles. The hard particles are preferably of less than 150 xcexcm of particle size.
In the iron-based sintered alloys mentioned above, solid lubricant such as fluoride (LiF2, CaF2, BaF2 and the like), boride (BN and the like) and the sulfide (MnS and the like) may be uniformly dispersed. The amount of the solid lubricant is from 1 to 20% by weight based on the iron-based sintered alloy, i.e., the total of the Fexe2x80x94Nixe2x80x94Crxe2x80x94C alloy and the solid lubricant, and occasionally the hard particles. The solid lubricant is preferably of less than 45 xcexcm of particle size.
A preferred method for producing the iron-based sintered alloy according to the present invention comprises the steps of:
preparing the raw material powder, which consists, by weight %, of from 0.5 to 5% of nickel (Ni), from 0.5 to 4% of chromium (Cr), from 0.5 to 2% of carbon (C) and the balance being iron (Fe) and unavoidable impurities by using at least an iron (Fe)-chromium (Cr) powder capable of supplying the total amount of chromium (Cr);
mixing zinc stearate and said raw material powder to prepare a green mixture;
pressing the green mixture to form a green compact;
heating the green compact to dewax; and,
sintering the green compact followed by cooling and then, annealing if necessary.
Preferably, the raw material powder consists of pure-iron (Fe) powder having average particle size of 75xcx9c150 xcexcm, iron (Fe)-chromium (Cr) alloy powder containing chromium (Cr) of from (10) to (14)% having average particle size of 75xcx9c106 xcexcm, nickel (Ni) powder having particle size less than 45 xcexcm and fine graphite (C) powder. The nickel powder is preferably pure nickel powder. The method may further comprise a step of mixing the raw material powder with from 3 to 20% of one or more hard particles selected from (1) hard particles which consist of from 50 to 57% of chromium (Cr), from 18 to 22% of molybdenum (Mo), from 8 to 12% of cobalt (Co), from 0.1 to 1.4% of carbon (C), from 0.8 to 1.3 of silicon (Si) and the balance being iron (Fe), (2) hard particles which consist of from 27 to 33% of chromium (Cr), from 22 to 28% of tungsten (W) from 8 to 12% of cobalt (Co), from 1.7 to 2.3% of carbon (C), from 1.0 to 2.0% of silicon (Si) and the balance being iron (Fe), (3) hard particles which consist of from 60 to 70% of molybdenum (Mo), 0.01% or less of carbon and the balance being iron (Fe), and (4) hard particles which consist of Stellite alloy, and/or with from 1 to 20% of solid lubricant, as well as with the zinc stearate, thereby preparing green mixture.
The composition of the iron-based sintered alloy according to the present invention is hereinafter described.
Nickel (Ni) is dissolved in the iron (Fe) matrix and enhances its strength and heat resistance. Wear resistance of the iron-based sintered alloy at the operation temperature of the valve is thus enhanced. The addition amount of nickel (Ni) is from 0.5 to 5%. When the addition amount of nickel (Ni) is less than 0.5%, the wear resistance is not satisfactorily improved. On the other hand, when the nickel (Ni) content is more than 5%, although the mechanical properties of the iron-based sintered alloy are excellent, the opposite material (valve) is seriously worn out (see examples No. 28 and No. 29), probably because the high Ni content of the valve seat results in disadvantageous adhesive wear condition with the valve which has high nickel (Ni) content to enhance the heat resistance. Such phenomenon is known as the sliding of materials of the same kind. In addition, when the nickel (Ni) content is more than 5%, the cost increases disadvantageously. The nickel (Ni) content is, therefore, from 0.5 to 5%, preferably from 1.5 to 3%.
The chromium (Cr) content is from 0.5 to 4%. When the chromium (Cr) content is less than 0.5%, the heat resistance and the oxidation resistance are not improved satisfactorily. On the other hand, when the chromium (Cr) content is more than 4%, the amount of carbides formed is so large that the machining of the iron-based sintered alloy are disadvantageously difficult, and, further, the alloy is embrittled.
In order to uniformly dissolve chromium (Cr) and disperse chromium carbides (CrxCy) in the iron-based matrix, iron-powder containing chromium (Cr) or iron (Fe)-nickel (Ni) powder containing chromium (Cr) can be used. For example, atomized iron-chromium powder and iron-nickel-chromium powder are commercially available. Such powder is expensive and cost reduction cannot be attained. Nickel (Ni) should, therefore, be used in the form of pure nickel (Ni) powder having preferably the particle size of less than 45 xcexcm.
When the chromium (Cr) in the form of metallic chromium (Cr) is added in the raw material powder the chromium (Cr) reacts with carbon (C) and forms large and hard carbides. In addition, since chromium (Cr) carbide has poor wettability with the iron-based matrix, there is a disadvantage that the opposite materials is attacked by the chromium carbides which work as abrasives. Desirably, the chromium (Cr) is preliminarily dissolved in the iron (Fe), and the so-prepared Fexe2x80x94Cr powder is used as the main material. Chromium carbides dispersed in the iron-based matrix are desirably as fine as (20) xcexcm or less in average.
Carbon (C) content is from 0.5 to 2%. When the carbon (C) content is less than 0.5%, ferrite (xcex1 solid solution) comes out and lowers the wear resistance. On the other hand, when the carbon (C) content is more than 2%, martensite and carbides are formed in excess so that the machining of the iron-based sintered alloy becomes disadvantageously difficult and such alloy is embrittled.
The content of carbon (C) is determined within the range of 0.5 to 2% taking the nickel (Ni) and chromium (Cr) contents and the kind and amount of the hard particles into consideration in such a manner that the ferrite and martensite in excess are not formed. Area % of ferrite should be 5% or less. Area % of martensite should be 20% or less.
The hard particles used occasionally has generally Hv 900 or more of hardness and has a particle size of 45 to 106 xcexcm.
Preferred hard particles are as follows.
(1) Hard particles which consist of from 50 to 57% of chromium (Cr), from 18 to 22% of molybdenum (Mo), from 18 to 12% of cobalt (Co), from 0.1 to 1.4% of carbon (C), from 0.8 to 1.3% of silicon (Si), the balance being iron (Fe).
(2) Hard particles which consist of from 27 to 33% of chromium (Cr), from 22 to 28% of tungsten (W), from 8 to 12% of cobalt (Co), from 1.7 to 2.3% of carbon (C), from 1.0 to 2.0% of silicon (Si) and the balance being iron (Fe).
(3) Hard particles which (consist of from 60 to 70% of molybdenum (Mo), 0.01% less of carbon (C) and the balance being iron (Fe).
(4) Hard particles which consist of Stellite alloy
The hard particles dispersed enhance the wear resistance of the valve seat by dispersion strengthening. The alloying elements of the hard particles diffuse from those particles and form a high-alloy layer around the particles. The wear resistance is, therefore, significantly improved. The amount of hard particles is from 3 to 20%. When the amount of hard particles is less than 3%, the wear resistance is not improved sufficiently. When the amount of hard particles is more than 20%, the wear resistance is not so improved commensurate with the amount. The iron-based sintered alloy is embrittled and involves, therefore, problems in strength and machinability. The opposite valve tends to be worn out greatly along with the increase of the amount of hard particles. The cost increases as well. From such several points of view, the amount more than 20% of hard particles is not preferable.
The present invention is characterized as compared with the prior application in the following points: (1) the wear resistance of a valve seat is maintained at a moderate level; (2) the wear of the valve seat and the valve, which are subjected to hammering and sliding action with respect to one another, is comprehensively improved; and, (3) the alloying elements of the iron matrix are decreased to the minimum level to reduce the cost.
The iron-based sintered alloy for use as a valve seat and its production method according to the present invention is explained with reference to the examples.